Hydrostatic pumps are devices that convert mechanical energy into fluid energy. Such pumps are typically used in combination with hydrostatic motors, with the combination being generally referred to as a variator as will be appreciated by those of skill in the art. Variators are employed in many power transmission or production devices and systems. An example of such a system is a continuously variable transmission (CVT).
This type of system can be seen in U.S. Pat. No. 6,385,970 to Kuras et al. In particular, this reference discusses the issue of lug or stall prevention. This condition occurs when the power demanded from the engine, e.g., because of a change in load or machine configuration, causes the engine to stall or severely slow down. The '970 patent notes that this condition may occur when the actuation of the hydrostatic elements of the system respond too slowly to effectively accommodate a changing environment. For example, if a bulldozer blade encounters sudden resistance, the power demand of the system will rise suddenly and dramatically, and the system may be unable to reduce the power demand in time to avoid a lug or stall condition.
The solution proposed in the '970 patent is an especially effective control scheme designed to provide a rapid response in the system elements. However, the system response frequency is ultimately still limited by its physical components. Thus, although the '970 solution eliminates much of the delay in the control system itself, the overall frequency response of the transmission will be hindered to some extent by the inertia and responsiveness of the system components such as the hydrostatic pump. In order to have a more accurately controlled transmission system, it is desirable to increase the responsiveness of the transmission and, in particular, the responsiveness of the hydrostatic pump.
Although the resolution of deficiencies of the prior art has been found by the inventors to be desirable, such resolution is not a critical or essential limitation of the disclosed principles. Moreover, this background section is presented as a convenience to the reader who may not be of skill in this art. However, it will be appreciated that this section is too brief to attempt to accurately and completely survey the prior art. The preceding background description is thus a simplified and anecdotal narrative and is not intended to replace printed references in the art. To the extent an inconsistency or omission between the demonstrated state of the printed art and the foregoing narrative exists, the foregoing narrative is not intended to cure such inconsistency or omission. Rather, applicants would defer to the demonstrated state of the printed art.